1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
A conventional image forming apparatus using an electrophotographic system includes a photosensitive drum serving as an image bearing member and a developing roller serving as a developer carrying member. In this image forming apparatus, a development process for visualizing a latent image formed on the photosensitive drum is performed by transferring toner serving as a developer carried on the developing roller to the latent image. In a region (referred to hereafter as a non-image portion) of the photosensitive drum where the toner is not to be transferred, within a contact region (referred to hereafter as a developing nip portion) where the photosensitive drum contacts the developing roller, a voltage is applied so that the toner receives a force traveling from the photosensitive drum toward the developing roller.
Here, non-image portion contamination (referred to hereafter as fog) may occur when the toner is transferred to the non-image portion of the photosensitive drum, where the toner is not intended to be transferred. Fog is generated when a charge of the toner decays or a polarity of the toner reverses in the developing nip portion where the photosensitive drum contacts the developing roller. It is known that a charge-providing performance in relation to the toner deteriorates particularly in a high humidity environment. When the charge-providing performance in relation to the toner deteriorates, the charge of the toner decays, leading to an increase in the amount of fog.
Japanese Patent Publication No. H7-31454 proposes setting a volume resistance of the developing roller at or above a predetermined value in order to suppress the occurrence of fog in which toner is transferred onto a non-image portion of a photosensitive drum.